The present application relates to spin-dependent tunneling (SDT) devices. Such devices may be employed in many applications, including information storage and retrieval devices (e.g., electromagnetic transducers), solid-state memory for computers and digital processing systems (e.g., MRAM) and measurement and testing systems (e.g., magnetic field sensors).
Spin-dependent tunneling (SDT) effects are believed to depend upon a quantum mechanical probability of electron tunneling from one ferromagnetic (FM) electrode to another through a thin, electrically nonconductive layer, with the probability of tunneling depending upon the direction of magnetization of one electrode versus the other. SDT effects have many potential applications in magnetic field sensing devices, such as magnetic field sensors and information storage and retrieval devices. Read transducers for magnetic heads used in disk or tape drives, which may be termed magnetoresistive (MR) sensors, and solid-state memory devices such as magnetic random access memory (MRAM), are potential commercial applications for spin tunneling effects.
Elements of SDT devices include two FM electrodes and an electrically insulating tunneling barrier. One of the electrodes may include a pinned ferromagnetic layer and the other may include a free ferromagnetic layer. The pinned layer typically consists of a FM layer that has its magnetic moment stabilized by a pinning structure. The pinning structure may be an antiferromagnetic (AFM) layer that adjoins the pinned layer. The magnetic stabilization of the pinned layer may also be accomplished with a synthetic AFM structure that includes a transition metal such as ruthenium (Ru) in a sandwich between two FM layers, in which the transition metal layer has a precisely defined thickness that is typically less than 10 xc3x85. The magnetization direction of the pinned FM layer is set upon deposition and annealing in a magnetic field. The free layer is typically a magnetically soft FM layer.
The free layer is designed to be magnetically decoupled from the pinned layer, so that the pinned layer does not hinder the response of the free layer to a magnetic field signal that is to be detected. The nonmagnetic tunneling barrier provides the magnetic decoupling between the pinned and free layers. The tunneling barrier is made of a thin dielectric layer, such as Al2O3 or AlN, which has a thickness typically in a range between 0.5 nm and 2 nm.
The tunnel barrier layer is designed to be a uniform and pinhole free dielectric film at the atomic scale, in order to avoid electrical shorting and ferromagnetic coupling through the pinholes. For applications involving tunneling magnetoresistive (TMR) heads, it is also desirable for the device resistance to be relatively low, in order to achieve a wide bandwidth and high frequency operation. The probability of electron tunneling through a tunnel barrier increases exponentially as the barrier is made thinner, however, for thicknesses less than 10 xc3x85 electrical shorting between the electrodes becomes increasingly problematic.
For example, a media-facing surface of MR sensors may be formed by lapping or polishing in a direction that traverses the tunnel barrier layer, which can cause dislodged electrode particles to bridge across a thin barrier. Similarly, conventional solid-state memory processing requires annealing at a relatively high temperature after formation of memory cells, which could in the case of MRAM devices cause diffusion of electrode materials into a tunnel barrier.
For a tunnel barrier material having a uniform specific resistance at each point, the overall resistance of the barrier layer is an exponential function of the thickness of the layer and inversely proportional to the area of the layer. For MR heads the area of the tunnel barrier layer is constrained, however, by the desired resolution of the head. Similarly, for MRAM applications the area of the tunnel barrier layer is constrained by the desired density of the memory cells.
The resistance and area product (RA product) is a figure of merit for SDT films, and is sensitively dependent upon the barrier thickness. Given the constraints upon the area of the devices, tunnel barrier layers may be as thin as several atomic layers. Another figure of merit for a SDT device is the magnetoresistance, which is the change in resistance divided by the resistance (xcex94R/R) of the device in response to a change in applied magnetic field. Since the noise of the device is related to the resistance, the magnetoresistance is also a measure of the signal to noise ratio (SNR) of the device.
In accordance with an embodiment of the present disclosure, a tunneling barrier for a spin dependent tunneling (SDT) device includes a ferromagnetic element dispersed substantially throughout the barrier in a minority concentration. The use of such a tunneling barrier has been found to increase the magnetoresistance, also known as the xcex94R/R response to an applied magnetic field, improving the signal and the signal to noise ratio. Such an increased xcex94R/R response also offers the possibility of decreasing an area of the tunnel barrier layer. Decreasing the area of the tunnel barrier layer can afford improvements in resolution of devices such as MR sensors and increased density of devices such as of MRAM cells.